"EndLayoutGroup: BeginLayoutGroup must be called first." error appears in the console when we try to create animation clip and then cancel creating it.

Steps to Reproduce:
1. Create a new project.
2. Open Animation Window
3. Select any GO.
4. Click "Create" button in Animation Window.
5. When "Create New Animation" window pops up, click "Cancel"...

error appears in console:

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEditor.SplitterGUILayout.EndHorizontalSplit () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/Splitter.cs:478)
UnityEditor.AnimEditor.OnAnimEditorGUI (UnityEditor.EditorWindow parent, Rect position) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimEditor.cs:223)
UnityEditor.AnimationWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimationWindow.cs:88)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:295)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:288)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:255)

Notes:
-Appears only on OSX.
-Appears every time.

Regression found on...
Version 2017.3.0b1 (bc2668834c45)
+ 2018.1

NOT repro on...
Version 2017.3.0a7 (2bdce6e797f4)
+ 2017.2.0p1

Please see attached video to reproduce the bug. (EndLayoutError(AnimationClip).mov)

License type: Pro